The standard telephone consists of a base unit and a handset unit connected to each other by an electrical cord. The base unit itself is connected by another cord to a receptacle on a wall, telephone pole or a similar immovable structure to which the telephone network line extends. Therefore, the range of movement of the operator of the telephone is quite limited. Even when the cords connecting the handset unit to the base unit and the base unit to the wall are long, it can be cumbersome to move either the entire telephone around to make calls from different locations or to walk around with the handset unit once a call has been placed. The simple fact that there is always a continuous physical connection between the person making the phone call and the immovable wall or other fixed structure can be a great inconvenience.
The cordless telephone represents a significant improvement over the standard telephone. In the conventional cordless telephone, the base unit is still connected to the receptacle on the immovable wall or the like by a cord so that message signals from the telephone network line may be received and transmitted. However, the handset unit of the cordless telephone is an independently operative unit from which calls may be made and by which calls may be received with no physical connection to the base unit. Instead, the base unit and the handset unit of the cordless telephone communicate with each other over a communication channel established by the transmission and reception of electromagnetic waves, conventionally radio waves. The handset unit may then be taken to distances of up to 300 meters from the base unit, while still making and receiving telephone calls.
The handset unit has a transmitting/receiving system or transceiver and a loudspeaker in the earpiece and a microphone in the mouthpiece. In some cordless telephones, the base unit also has either loudspeaker and microphone incorporated within the base unit itself, or in an auxiliary standard telephone connectable to both the base unit and the outside telephone network line. When the base unit receives an incoming call, the base loudspeaker/microphone can be used to converse directly with the caller. Frequently such a cordless telephone has a transfer capability wherein the incoming call is transferred from the base unit to the handset unit to establish communication between the handset loudspeaker/microphone and the telephone network line through the base unit, with the base loudspeaker/microphone or auxiliary telephone being no longer in communication with the telephone network line. By this means, an operator at the base unit can receive an incoming call and can transfer it to a second operator using the handset unit. This is an additional feature making the cordless telephone even more convenient.
However, previously proposed cordless telephones having this transfer capability have required a number of extra switches, so that the proper operation thereof can be quite complicated. In one such cordless telephone, when an incoming call is received, the operator at the base unit depresses a hold key on the base unit to hold the outside line, and then moves a mode switch to establish an intercom mode wherein the base loudspeaker/microphone is in communication with the handset loudspeaker/microphone. Then the base operator depresses a call key on the base unit to send a signal to the handset unit to cause it to ring. If the handset operator answers by depressing a key on the handset unit, the base operator returns the mode switch from the intercom position and the hold key is again depressed to release the incoming call, whereby the handset loudspeaker/microphone receives the incoming call. On the other hand, if the operator at the handset unit does not answer, the mode switch and hold key are actuated in turn so that the loudspeaker/microphone of the base unit again communicate with the incoming call.
This cordless telephone requires at minimum a hold key, a mode switch, and a call key on the base unit. The higher the number of additional switches and keys, the greater is the consequent complexity and manufacturing cost of the cordless telephone. Furthermore, these switches and keys must be exposed for easy access on the surface of the unit and thus there is a possibility that they may be operated at the wrong time, in the wrong order, or even inadvertently, with the result that the incoming call may not be transferred to the handset unit, or in the worst case the incoming call will be cut off.
Furthermore, there is no provision for transferring the call back from the handset unit to the base unit, which would be a highly advantageous feature adding considerably to the convenience of the cordless telephone.
Additionally, the previously proposed cordless telephones with transfer capability have not had the capacity to establish three way communication between the handset loudspeaker/microphone, the base loudspeaker/microphone, and the telephone network line.